When puppies are born, their immune system is not 
		fully developed.  This would normally leave the puppy open to many 
		forms of infection.  But, thanks to maternal antibodies, these 
		puppies get immunity from their mother while in the uterus and also from 
		the first couple days of her milk (colostrum).  This is crucial for their 
		survival.
		How long those antibodies last in any given puppy 
		varies from individual to individual, even within the same litter.  
		Maternal antibodies against different diseases even wear off at 
		different times within the same puppy.  
		While maternal antibodies are present in the puppy, 
		any vaccines that are given to that puppy will be inactivated by Mom's 
		antibodies without ever stimulating the puppy's own immune system.  
		Because of this fact, we, as veterinarians, give a series of vaccines in 
		order to stimulates that puppy's own antibodies.  The problem is 
		that we don't know when each puppy will become susceptible to the 
		vaccine.  Unfortunately, they must be susceptible to the virus in 
		order for the vaccine to be effective (because that is when Mom's 
		immunity wears off).  The goal then becomes to get the vaccine in 
		the puppy before he/she encounters the virus. 
		We do know that at 16 weeks of age, maternal 
		antibodies in 95% of all puppies are gone.  That means that if you 
		vaccinate your puppy at 16 weeks of age, there is a 95% chance that he 
		is covered for those viruses you vaccinated against.  However, when 
		you vaccinate against a specific disease for the first time, even in an 
		adult animal, it is best to give at least two vaccinations.  This 
		is because the second vaccination will produce a much greater response 
		if it is following a vaccine given 2-4 weeks earlier.  (The first 
		one sort of "primes" the system.)
		So you ask, "Why don't we pull blood and check the 
		puppy's antibody titers, to see when they need to be vaccinated?" 
		Answer:  Economics!  It actually cost more to pull blood, send 
		it off to the lab, wait on results, continue to re-check levels every 
		couple of weeks, and then vaccinate when the titers drop.  Other 
		than that -- there is nothing wrong with that approach.